The leading cause of death in the world today is water-borne disease. Over 2.3 billion people worldwide suffer from diseases linked to water, with 300 million people critically ill and 2.2 million people dying every year, mostly children below five years of age1. The majority of these people are living on less than two dollars per day, severely limiting the range of technologies they can afford for water purification. 1United Nations Task Force on Water and Sanitation 2003
At the same time, nations worldwide are using greater percentages of their freshwater resources every year, reducing the availability of safe drinking water and increasing the need for effective water purification systems. The United Nations Population Fund projects that in 2025, if present rates of water consumption are maintained, 5 billion of the world's projected 7.9 billion person population will live in areas where safe water is scarce.
Much of the unmet need for drinking water occurs in developing countries. The average per capita water consumption in the developing world is 2 gallons (7.6 liters) per person per day. For a typical 1000 person village in the developing world, the drinking water requirement is 2000 gallons (7600 liters) per day. Many existing UVC water purifying systems are designed for individual users or for small groups of people and therefore have too low of flow rate to supply this daily volume of water. The price per gallon of many of these small systems is too high to make them practical for use in developing countries.
Other water purification systems are intended to be used as fixed infrastructure for urban areas with much larger water volume and are often too expensive to be deployed in rural areas, especially since the rural areas in the developing world often lack the reliable electric power required for these large plants to operate. The present invention is intended for areas where large metropolitan water purification plants are cost ineffective. One embodiment of the water purification system is portable and lightweight, allowing it to be easily transported to remote areas or to be used in disaster relief or to be rapidly deployed in emergencies.
Existing Drinking Water Purification Techniques and Systems
Many natural drinking water sources are contaminated with waterborne pathogens. Lakes, rivers, and streams worldwide commonly contain bacteria, viruses, and protozoa which can cause serious health problems. Even dug wells and tube wells, which are commonly used as drinking water sources worldwide, can be contaminated by groundwater during the rainy season in many parts of the world when surface water contaminates the well introducing waterborne pathogens. If the tube well is poorly constructed or poorly maintained, contaminated groundwater can seep into the well on a more regular basis.
Many water purification techniques exist ranging from simple and inexpensive to very sophisticated and expensive, with a resulting wide range of levels of effectiveness in treating waterborne pathogens, organic contaminants, and inorganic contaminants. One simple and common technique is to boil the water. For many parts of the world, boiling is impractical for everyday use because of the significant amount of energy required and because of the possibility that the water will not be heated to a full boil or heated for too short a time so as not to inactivate or kill the waterborne pathogens. Boiling also increases the concentration of heavy metals already present in the water due to water loss during the boiling process.
A more recently developed technique is solar water disinfection, or SODIS. Unlike boiling, SODIS relies only on solar energy to disinfect the water. SODIS is a simple method to inactivate or kill pathogens using a combination of solar heat and sunlight. SODIS is used with 1 to 2 liter plastic bottles, preferably made of polyethylene terephalate and preferably painted black on the non-sunlit back surface of the bottles. The bottles are completely filled with water and placed on a corrugated steel sheet in the sun. SODIS requires the water to attain a temperature of 60° C. to 80° C. for a minimum of 4 hours to remove the pathogens. Under cloudy conditions, the bottles must be placed in the sun for two consecutive days. SODIS is very inexpensive to implement, but is not as effective against viruses and protozoa. SODIS processed water is not recommended for infants less than 18 months or for people with chronic gastrointestinal illness. The quality of the purified water is very difficult to control. The technique does not work as well with even partial shade. SODIS does not kill protozoa such as cryptosporidium parvum oocysts.
Other more advanced water purification systems are readily available but have limitations as well. Both iodine and chlorine are effective at eradicating most bacteria, viruses, and protozoa. However, cryptosporidium parvum is one of several chlorine-resistant pathogens which is increasing in importance. Cryptosporidium parvum is an intestinal parasite that can be life threatening to infants, the elderly and people with compromised immune systems. Typically, it takes about seven days for symptoms of cryptosporidiosis to appear, long after the initial exposure occurred. The illness often can last up to two weeks. Removing protozoa like cryptosporidium parvum oocysts and giardia with chlorine purification is difficult because it requires a high product of chlorine concentration and application time. Since adding too much chlorine to drinking water can cause organ damage or death in humans, the concentration of chlorine that can be used to disinfect the water is limited. Therefore, the time required for chlorine disinfection of cryptosporidium is often prohibitive.
Chlorine has been shown to produce hazardous trihalomethanes when it is added to water with organic contaminants, as is typically found in natural sources such as rivers, lakes and streams. Trihalomethanes are also environmental pollutants, and many such as chloroform are considered carcinogenic. Additionally, chlorine is ineffective if the pH of the water is below 7.5. If the chlorine is from a bleach bottle more than six months old, it loses its potency.
Both iodine and chlorine can cause side effects in humans if used for an extended time. Iodine treated drinking water is not suitable for pregnant women or women over age 50 or people with thyroid problems.
Many modern water purification systems use chloramines instead of chlorine, adding increased sophistication to the treatment systems.
Chlorine dioxide is also used as a purification agent that kills most bacteria, viruses and protozoa. Due to the explosion hazard, it is typically manufactured at the point of use, increasing purification system complexity and expense. Chlorine dioxide purification produces reaction by-products, the toxicity of which is unknown.
Ozone is the most effective disinfectant for all types of pathogens in drinking water. It leaves minimal or no residue in the water. However, ozonation systems are expensive to implement.
Other approaches rely on advanced ceramics or membranes instead of disinfectants to filter pathogens from the water. Ceramic filters are effective for filtering protozoa, but may clog easily due to particulates in the water. Typical ceramic filter elements have pores from 2 to 5 microns in size. Since bacteria such as cholera and salmonella are typically between 0.2 and 1.0 microns in size, bacteria pass through many of these filters. Viruses such as Hepatitis A and B, rotavirus, and the Norwalk virus are typically below 0.004 microns in size, allowing them to pass easily through the ceramic filter element. These viruses and some bacteria may even penetrate reverse osmosis purifiers.
Reverse osmosis (RO) water purifiers are capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, heavy metals, chlorine and related byproducts, and other contaminants with a molecular weight greater than 150-250 daltons. The reverse osmosis systems require pressurized water that is not available in many parts of the developing world. Reverse osmosis membranes may foul unless the incoming water is carefully filtered before the reverse osmosis system. The RO systems may also need water softening equipment upstream of the RO purifier where the water has high mineral content (hard water) to prevent membrane fouling.
There are two primary types of RO membrane: Thin Film Composite (TFC) and Cellulose Triacetate (CTA). TFC membranes filter out more contaminants than CTA membranes, but they are more susceptible to damage by chlorine. Since the RO membranes are subject to degradation by chlorine, iron, manganese, hydrogen sulfide, and to bacterial attack, a sediment filter and a granular activated carbon (GAC) pre-filter is often used ahead of the RO system. Additional treatment such as GAC is needed for volatile organic compounds such as benzene, MTBE, trichloroethylene, trihalomethanes, and radon.
The RO process is fairly slow and may require from 3 to 10 gallons (11.4 to 38 liters) of untreated water for each gallon (3.8 liters) of purified water, making it problematic for use in areas where water is scarce. RO water treatment is not recommended for use without secondary treatment such as UV treatment for water that may contain biological contaminants such as viruses and bacteria.
UVC purifiers work by irradiating the pathogens in the water, usually with low pressure mercury lamp(s) which emit a 253.7 nm peak wavelength. Other UVC systems are based on medium pressure mercury lamps. Many different types of UVC water purification systems currently exist.
UVC has a wavelength ranging from approximately 200 nm to 280 nm and is also called germicidal UV because of its proven effectiveness in inactivating or killing a very wide range of viruses, bacteria, protozoa, helminthes, yeast, and mold. An advantage of UVC purification systems is that they are capable of treating the drinking water for all segments of the population, unlike other disinfection technologies such as iodine and chlorine. UVC systems do not leave residual disinfection compounds in the water.
One of the most difficult pathogens to kill is the cryptosporidium parvum oocyst, which requires a UV-C irradiation density of approximately 200 mJ/cm2 to kill.
Accordingly, what is needed is a water purification system which purifies all the water which is being treated, which requires no expensive chemicals or filters which will need to be replaced, which produces water which can be drunk by all persons, including pregnant women, small children and seniors and which allows the inexpensive adjustment of the purification process in response to the types and concentrations of impurities present in the water.